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Why is iron and carbon used to make Japanese knives?

Hello hello. I feel like it's been forever since I've posted factual/historical information, but I'm going to be honest... I've been slacking in my researches. I'm sorry. We're currently working on the knife catalog and looking for additional information to add to it. I thought reasoning behind the materials used to forge Japanese knives to be really interesting, so I decided to share.

Why is iron used to make knives?

Natural resource availability. When knives began to be produced in Japan, carbon was difficult to find, but Japanese iron was readily available and relatively inexpensive. Forgers cut the amount of carbon needed by forging the two metals together.

It is very difficult to sharpen something as hard as carbon. It was not very user friendly.

Carbon will bend and warp after being forged due to the nature of the metal. Forgers resolved this problem by adding a thin strip of carbon to a thick piece of steel. Even if a knife is purchased in good straightened condition, both metals will naturally want to bend in opposite directions over time. This is why it is important to have Japanese knives straighten out by a professional every now and then.
Side notes:
* If not hammered together properly, the iron and the carbon with separate.
[This one was separating while it was being hammered. You can usually tell if a knife is separating even after you purchase it, by looking at the bottom.]
* According to Mr. Sugai, ideally you want to leave the forged metals to rest for 60 years before going forward and polishing/sharpening/etc the blade.

The cooling process is faster when carbon is added. The faster cooling process helps forgers create the same product with similar characteristics.

Carbon is extremely hard, rigid and brittle. Forgers make kasumi and hongasumi knives for a more user friendly and flexible knife by adding soft iron to the carbon.

Thanks for reading!

Ps. I will be in Japan and China from April 10 to May 2. I will still check KKF, but please excuse me if my responses are a bit slow.

Indeed. Also, 60 years rest? If that were adhered to, most polishing/sharpening would done by the forger's children and grandchildren.

Nope, this part is supposedly true. Of course no one adheres to that, because its impractical and it would make Japanese knives absurdly expensive. But ideally you want the steel to rest for several years. I think Mr. Sugai may have been exaggerating when he said 60, but he did point out that a well made old Japanese knife is less likely to warp and bend overtime. I need clarification on how exactly this works. Bare with me on this one.

Nope, this part is supposedly true. Of course no one adheres to that, because its impractical and it would make Japanese knives absurdly expensive. But ideally you want the steel to rest for several years. I think Mr. Sugai may have been exaggerating when he said 60, but he did point out that a well made old Japanese knife is less likely to warp and bend overtime. I need clarification on how exactly this works. Bare with me on this one.

It is a real phenomenon called age hardening. In this case it stress relieves. Another example of it is old timer machinists preferring to machine "seasoned" cast iron engine blocks that had sat outside for at least a year.

It is a real phenomenon called age hardening. In this case it stress relieves. Another example of it is old timer machinists preferring to machine "seasoned" cast iron engine blocks that had sat outside for at least a year.

-AJ

-AJ

OOO. Okay. Thank you so much for the clarification. I was trying to figure out how to explain it.